Question

Помогите сам не справлюсь!

Answer 1

$\displaystyle\bf\\a_{1} =5\\\\a_{2} =7,5\\\\S_{50} =?\\\\\\a_{2} =a_{1} +d\\\\d=a_{2} - a_{1} =7,5-5=2,5\\\\a_{50}=a_{1} +49d= 5+49\cdot 2,5=5+122,5=127,5\\\\\\S_{50} =\frac{a_{1} + a_{50} }{2} \cdot 50=(a_{1} + a_{50} )\cdot 25=(5+127,5)\cdot 25=\\\\\\=132,5\cdot 25=3312,5\\\\\\Otvet \ : \ S_{50}=3312,5$